There is considerable interest in developing and using materials capable of absorbing or adsorbing contaminants from contaminated fluid media. Such contaminants include organic materials as well as ionic materials including heavy metals. Examples of contaminated fluid media include but is not limited to aqueous media such as waste water, industrial waste water, potable water or natural water systems such as lakes, rivers, seas and the like which have become naturally or artificially contaminated. It is very important, for obvious reasons, that such materials are not prohibitively expensive and that they can be used in relatively simple, cost-effective ways.
A good illustrative example is the removal of contaminating oils from waste water, harbour water, water from road run-off etc. In such cases, much of the oil contaminant floats on the water surface and can be removed either centrifugally or even more simply in a standard separator device. Such devices are well-known to those familiar with the techniques involved in water-cleaning. However, it is difficult to remove the final few percent of the oil using such methodology, and some of the oil inevitably becomes emulsified into the water, and cannot be removed in such a straightforward physical manner. In order to remove the last part of contaminating oils from water, it is usually necessary to expose the water to some type of sorptive material, in the form of a filter, filter cartridge or other suitable device.
“Sorptive materials” or “sorbants” are materials having absorbing and/or adsorbing properties. Commonly used sorbants are inorganic powders with hydrophobic surfaces, glass and silica beads and “resins”, activated carbons, and even simple hydrophobic materials such as polypropylene. The more effective sorptive materials such as activated carbons are expensive and difficult to handle.
A further example is the removal of dissolved heavy metal cations from waste water or other water streams. The most common methods of treatment are either via precipitation of the metals as insoluble salts, or by the use of an ion exchange resin or material that actively removes the ions from solution by chemical sorption. Ion-exchange resins are very expensive and the high price of such materials means that they have to be re-used or recycled many times to justify utilization. Additionally, if the aqueous medium to be treated contains other contaminants, particularly organic materials such as oils, then the performance of ion exchange resins can be impaired. In that case the efficiency of the ion exchange resin decreases, the necessary treatment time per liter water increases and the treatment becomes very expensive. Precipitation technologies are cheaper, but often require long treatment times and extensive use of settlement and sedimentation tanks.
The term “lignocellulose” means any of several closely related substances constituting the essential part of woody cell walls of plants and consisting of cellulose intimately associated with lignin and hemicellulose.
The terms “lignocellulose fibres” and “lignocellulosic fibres” are recognized by those skilled in the areas of natural product and plant sciences to mean fibres isolated from wood or other fibrous plant materials. Examples of plant materials having a great potential as a source of lignocellulosic fibres are wood, including soft and hard wood, flax, hemp, jute, coconut, cereal grasses and straws. As the name implies, lignocellulose fibres are natural fibres rich in the natural polymers cellulose, lignin and hemicelluloses. These materials are characterized and known to be extremely rich in hydroxyl groups which are functional chemical groups which are extremely sorptive towards water and other polar solvents due to its inherently polar nature. The lignocellulosic fibres are therefore also very sorptive towards water and are notably hygroscopic. The natural structure of the fibres is in the form of an elaborate capillary network of tubes (the empty cell lumina) and micro tubules (cell-wall pores), which creates an excellent sorptive matrix. These factors, coupled with the low cost and renewable, biodegradable nature of lignocellulosic fibres, makes them attractive candidate materials for use in water clean-up applications.
The hydroxyl groups within the fibres, and especially those located on the fibre surface, are also chemically reactive and can be readily chemically modified via reaction with chemical species which are known to react with hydroxyl groups. For example, such groups are readily esterified using reagents such as organic acid anhydrides, organic acid chlorides, well known to chemists familiar with the esterification of alcohols and other materials containing hydroxyl groups. It is known that lignocellulosic fibres, especially wood derived fibres, can be reacted with acid anhydrides to produce fibres that are chemically modified by esterification with non-polar acid anhydrides such as acetic and propionic anhydrides, and this is used to produce fibres that are more water stable for utilization in water-resistant fibreboard type composite products.
The interesting factor is in the present context is that the fibres are of course rendered partially hydrophobic by such a modification, and what was previously a fibrous sorptive matrix with an affinity for water can be transformed, in a relatively straightforward manner, into a sorptive matrix with an increased affinity for hydrophobic liquids and a much reduced affinity for water.
An example of the above mentioned modified lignocellulosic fibre material is described in GB Patent Application 2 248 610 A disclosing a method of absorbing hydrophobic water-immiscible liquids by treating the liquid with a modified lignocellulosic plant material in which hydroxyl groups have been esterified with an aliphatic monocarboxylic acid with 1-4 carbon atoms to render it relatively more absorbent to hydrophobic water-immiscible liquids. The modified fibres are used to absorb oils and related hydrophobic liquids from natural waters and waterways including rivers, lakes, seas, harbours etc. The modified fibres are not able to effectively break emulsions and have therefore limited potential in that they only achieve little more than physical separators.
U.S. Pat. No. 4,804,384 (Rowell et al.) discloses reaction of lignocellulosic material with uncatalyzed acetic anhydride in the absence of any cosolvent. The purpose is to improve dimensional stability and resistance to biological attack of the lignocellulosic material. Thus use as a sorptive material for the removal of oils, metal ions and other contaminants from a fluid medium is not suggested by Rowell.
As appears, there is still a need for a non-expensive sorptive material with improved ability to remove oils and other hydrophobic contaminants from aqueous media and with the additional ability to remove other contaminants from the aqueous media such as ionic materials including heavy metals.